Generally, a brushless DC (BLDC) motor is a type of a DC motor that does not use mechanical contacting units such as a brush and a commutator. The BLDC motor is known to use an electrical rectifier in lieu of the mechanical contacting units. A typical BLDC motor consists of two main portions, namely, a stator and a rotator. The stator is formed of a plurality of coils, which receive poly-phase currents (e.g., three-phase currents). Rotating magnetic fields are formed by the currents applied to the coils. The rotator is formed of at least one permanent magnet surrounded by the stator, wherein the rotator rotates within the stator.
To rotate the BLDC motor continuously, the rotating magnetic fields must be formed continuously in the BLDC motor. More specifically, in order to rotate the rotator, a commutation of currents applied to the coils must be performed at a proper timing. Commutation means that the currents applied to the coils are switched. In other words, the direction of the currents becomes changed. For example, three-phase currents are commutated in sequence in accordance with the angular positioning of the rotator. Therefore, the angular positioning of the rotator must be measured in order to perform a proper commutation. For this purpose, a position sensor, such as a hall sensor or a resolver, is required.
The BLDC motor has certain advantages since there is no need to replace a worn brush with a new one, the noise caused by an Electromagnetic Interference (EMI) is reduced, the heat transfer characteristic is fair, and a higher power can be obtained compared to other motors having the same size.
FIG. 1 is a block diagram showing a conventional BLDC motor system. The conventional BLDC motor system comprises a power supply 10 for supplying AC power, a converter 20 for converting AC power into DC power, an inverter 30 for performing a switching operation and outputting three-phase currents, a current sensor 40 for detecting phases of the currents outputted from the inverter 30, a BLDC motor 50, a stator 51 in the BLDC motor 50, a rotator 52 which is rotated by the switching operation of inverter 30, a position sensor 60 for detecting the position of a rotator 52, a speed sensor 70 for detecting the rotational speed of the rotator 52, a controller 80 for controlling the switching period of the inverter 30 with current values from the current sensor 40 and signals from the position sensor 60, and a speed sensor 70 for performing a commutation at a proper timing.
Generally, a rectifier circuit, which simply converts AC power into DC power, may be used as the converter 20 for driving the BLDC motor 50.
The inverter 30, which includes six switching elements, is a power transducer for converting DC power into three-phase AC power. There are two types of inverters, i.e., a current source inverter and a voltage source inverter. The voltage source inverter is mainly used for driving the BLDC motor 50. That is, the voltage source inverter receives DC power and applies voltages to the BLDC motor 50 in a form of a pulse string.
The controller 80 controls the application of the three-phase currents from the inverter 30 to the stator 51 for rotating the rotator 52. This is accomplished by synchronizing the magnetic poles of the rotator 52 with the magnetic poles of the stator such that the operation of the BLDC motor 50 is controlled. More particularly, the controller 80 controls the operation of the BLDC motor 50 by applying a voltage and a current in the form of pulses, the widths of which are modulated with saw-tooth wave signals inputted from the outside. The controller 80 detects the current applied to the BLDC motor 50, generates a control signal by calculating the signals inputted from the position sensor 60 and the speed sensor 70 based on the value of the detected current, and supplies the control signal to the inverter 30. Such control by the BLDC motor is often called the torque control since the BLDC motor 50 controls the torque, which occurs by the current supplied to the BLDC motor 50.
According to the conventional method adopting the switching operation of the inverter 30, the currents inputted to the BLDC motor are controlled through the use of the three-phase currents outputted from the inverter 30 for reducing a torque ripple. That is, the switching elements of the inverter 30 are used to apply three-phase voltages to the BLDC motor 50. In such a case, the BLDC motor 50 is controlled by a constant duty ratio of pulses for switching. In other words, the current of each phase is applied to the coil whenever the rotator is detected to be at a predetermined position, for example, at an angle of 120° from a reference point. Also, it takes time to increase the current to a desired value or to decrease the current to zero. That is, the increase or decrease of the current is delayed due to a resistor and an inductance of each coil. The current delay, as well as the changes of a rotational angle, generates torque. The torque ripple is generated by the commutation whenever the phase of the current applied to the coil is changed.
The torque ripple causes vibrations and noises in the motor. To reduce the torque ripple in the BLDC motor 50 using a trapezoid wave of counter-electromotive force, it is preferable to use three AC current sensors 40 for controlling the currents outputted from the inverter 30. There is currently a method of reducing the torque ripple using a single DC current sensor for controlling the output currents of the inverter instead of using three current sensors.
FIG. 2 is a graph showing the saturation conditions of currents and voltages in the current controller 80 during the commutation interval of the conventional inverter through the use of a saw-tooth wave as a carrier wave for the Pulse Width Modulation (PWM). A DC link current “|id|” is a value of the current outputted from a DC output port (not shown) of the BLDC motor 50 through each coil having a different phase. A reference current “idref” is a current inputted to the controller 80 from the outside. A DC supplying voltage “Vd” is a voltage outputted from the converter 20. A commutation interval “tc” is a time when the commutation occurs in the BLDC motor 50. A sampling time “Ts” is duration of time required to control the current. “Vmout” denotes a voltage obtained by performing a deadbeat current control. The deadbeat current control is performed when an error occurs between the current value |id| of each phase and the reference current value idref. The controller 80 performs the deadbeat current control to improve the current control performance. As shown in FIG. 2, when the deadbeat current control is performed, it is observed that voltage Vmout outputted from the controller 80 is saturated during commutation interval tc of the inverter 30. The current value |id| decreases to zero at the moment of initializing the commutation, and then starts to increase from zero. This occurs due to the characteristic of the BLDC motor system. More specifically, such occurrence is due to the current flowing through the inner diodes of switches during the commutation. In this case, the current value |id| should have a normal value necessary for operation. Thus, the controller 80 outputs the voltage Vmout to perform the deadbeat current control for restoring the value of current |id|. The zero-point reach time of one current (e.g., a-phase current) does not coincide with a final-point reach time of another current (e.g., b-phase current) in one commutation interval due to the saturation of output voltage Vmout. As such, the torque ripple occurs in the BLDC motor 50 during the commutation.
According to the conventional methods for reducing the torque ripple using a single DC current sensor, the switching elements of the inverter 30 are switched with a constant duty ratio of pulses during the commutation interval, or the turn-off time of the switching elements of the inverter 30 is delayed. However, the conventional methods are disadvantageous since they are undesirably sensitive to the change of parameters of the BLDC motor. Thus, the stable performance of the BLDC cannot be guaranteed at the low or high speed due to the speed change of the BLDC motor 50.